Laser diode

ABSTRACT

A laser diode ( 1 ) for outputting light of a single mode comprises a substrate layer ( 5 ), a cladding layer ( 6 ) and a compound light propagating layer ( 7 ) comprising first, second, third and fourth layers ( 9  to  12 ). A wave guiding region ( 15 ) of refractive index lower than its adjacent regions is defined by the third layer ( 11 ) by two quantum wells ( 16 ) positioned at the anti-node of light of a single mode which is propagating in the third layer ( 11 ) for propagating and amplifying the single mode light. A central ridge ( 17 ) locates the wave guiding region transversely of the direction of light propagation. The wave guiding region ( 15 ) can also be defined by shaping the top cladding layer ( 6 ) by forming an elongated central channel through the central ridge ( 17 ).

This is a National stage entry under 35 U.S.C. §371 of Application No.PCT/EP01/00040 filed Mar. 28, 2001; the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a laser diode, and in particular, to asemiconductor laser diode for providing light of a single spatial mode.The invention also relates to a method for providing such a laser diode.

BACKGROUND TO THE INVENTION

In optical technology there is a requirement for high power laser diodeswith relatively stable single spatial mode bright light outputcharacteristics. Brightness is defined as the optical output power perunit emitting area into unit solid angle from a spatially coherentsource with a single lobed emission pattern. Brightness determines theminimum spot size to which light from a source can be efficientlyfocused. Accordingly, where efficient focusing of laser light is arequirement, for example, in cutting and welding of metal, brightness ofthe light output of a laser diode is an important property. Furthermore,the brightness of light outputted from a laser diode determines theextent to which the laser output may be coupled to an optical fibre ofrelatively small diameter. In general, in known high power laser diodes,an increase in optical power output can only be achieved at the expenseof emission brightness or in the stability of the light output which ingeneral results in increased beam wander. Single spatial mode waveguidelaser diodes provide a relatively stable beam. However, due to the smallemission aperture of such laser diodes, the maximum optical power whichcan be emitted is limited due to the possible onset of facet damage.Unfortunately, while simply increasing the emitting aperture permitshigher optical power output, it also results in general, in loss ofspatial coherence and brightness. Broad area laser diodes withrelatively large emitting apertures which provide higher optical poweroutput also suffer from the disadvantage of lasing in higher order modesand/or a decrease in spatial coherence as a result of the onset offilamentation, and thus the maximum brightness which would otherwise beachieved is limited. Nonetheless, even in broad area laser diodes theoptical power output before the onset of optical damage to the laserdiode is still limited by optical absorption at the facet. Therequirement for spatial coherence in high optical power output laserdiodes and the danger of optical damage in narrow aperture laser diodeshave been limiting factors in the provision of high optical power outputlaser diodes for emitting a high intensity optical beam which can befocused to a micrometer size spot of the type typically required forefficient coupling to optical fibres for fibre amplifier applications,and for the production of high optical power densities for cutting,welding and soldering.

There is therefore a need for a laser diode and a method for providingsuch a laser diode which overcomes these problems.

The present invention is directed towards providing such a laser diode.

SUMMARY OF THE INVENTION

According to the invention there is provided a laser diode foroutputting light of a single spatial mode, the laser diode comprising asubstrate layer, a cladding layer, and a light propagating layer locatedbetween the substrate layer and the cladding layer, wherein a means isprovided for defining a wave guiding region in the light propagatinglayer, the defined wave guiding region being of refractive index lowerthan an adjacent region in the light propagating layer and being definedin the light propagating layer in a region in which light of the singlemode propagates, so that when the defined wave guiding region is pumpedthe intensity of the light of the single mode is amplified above lightof other modes propagated in the light propagating layer.

In one embodiment of the invention the means for defining the waveguiding region comprises a gain means located in the light propagatinglayer.

In another embodiment of the invention the gain means comprises at leastone quantum well located in the light propagating layer. Preferably, thegain means comprises at least two quantum wells located in the lightpropagating layer. Advantageously, the gain means comprises a pluralityof quantum wells located in the light propagating layer.

In one embodiment of the invention the gain means is located in thelight propagating layer adjacent an anti-node of the single mode light.

In another embodiment of the invention the wave guiding region isdefined in the light propagating layer during growing of the layers ofthe laser diode.

In a further embodiment of the invention a locating means is providedfor locating the wave guiding region in a direction transversely of thedirection of light propagation. Preferably, the locating means comprisesa longitudinally extending central locating ridge formed on the claddinglayer extending in the direction of light propagation for locating andstabilising the wave guiding region in the direction transversely of thedirection of light propagation. Advantageously, an electricallyconductive layer is provided on the central locating ridge forfacilitating pumping of current through the wave guiding region.

In an alternative embodiment of the invention the means for defining thewave guiding region comprises a means in one of the layers adjacent thelight propagating layer for reducing the refractive index in a region inthe light propagating layer for forming the wave guiding region.

In one embodiment of the invention the means in one of the layersadjacent the light propagating layer for reducing the refractive indexin a region in the light propagating layer for forming the wave guidingregion comprises a formation in the one of the said layers. Preferably,the formation in the layer adjacent the light propagating layercomprises a longitudinally extending central channel through the saidlayer extending in the direction of light propagation in the lightpropagating layer for defining the wave guiding region. Advantageously,the central channel defining the wave guiding region is formed in alongitudinally extending central locating ridge which extends in thedirection of light propagation. Preferably, the formation is formed inthe cladding layer.

In one embodiment of the invention an electrically conductive layer isformed on the formation in the cladding layer defining the wave guidingregion for facilitating pumping of the wave guiding region.

In another embodiment of the invention the means in one of the layersadjacent the light propagating layer for reducing the refractive indexin a region in the light propagating layer for forming the wave guidingregion comprises a portion which is implanted in the one of the saidlayers such that the implanted portion is of reduced refractive index tothat of the said one of the said layers. Preferably, the implantedportion in the said one of the said layers is electrically conductive.Advantageously, the implanted portion of the said one of the said layersis an elongated portion and extends longitudinally in the direction oflight propagation in the light propagating layer for defining the waveguiding region. Preferably, the said implanted portion is formed in thecladding layer.

The invention also provides a method for outputting light of a singlespatial mode from a laser diode, wherein the laser diode comprises asubstrate layer, a cladding layer, and a light propagating layer locatedbetween the substrate layer and the cladding layer, wherein the methodcomprises defining a wave guiding region in the light propagating layer,the defined wave guiding region being of refractive index lower than anadjacent region in the light propagating layer and being defined in thelight propagating layer in a region in which light of the single modepropagates, and pumping a current through the defined wave guidingregion so that the intensity of the light of the single mode isamplified above light of other modes propagated in the light propagatinglayer.

In one embodiment of the invention a gain means located in the lightpropagating layer for defining the wave guiding region.

In another embodiment of the invention the gain means is provided by atleast one quantum well located in the light propagating layer.

In one embodiment of the invention the wave guiding region is located ina transverse direction relative to the direction of light propagation.Preferably, the wave guiding region is located in the directiontransversely of the direction of light propagation by a central locatingridge formed on the cladding layer.

In an alternative embodiment of the invention the wave guiding region isdefined by providing a means in one of the layers adjacent the lightpropagating layer for reducing the refractive index in a region in thelight propagating layer for forming the wave guiding region.

In one embodiment of the invention the means provided in the said one ofthe layers adjacent the light propagating layer for reducing therefractive index in a region in the light propagating layer for formingthe wave guiding region is provided by a formation being provided in thesaid one of the said layers.

In another embodiment of the invention the means in one of the layersadjacent the light propagating layer for reducing the refractive indexin a region in the light propagating layer for forming the wave guidingregion comprises a portion which is implanted in the one of the saidlayers such that the implanted portion is of reduced refractive index tothat of the said one of the said layers.

Advantages of the Invention

The advantages of the laser diodes according to the invention are many.A particularly important advantage of the laser diodes according to theinvention is that they provide a high power light output of singlespatial mode which is stable. Additionally, the laser diodes accordingto the invention permit the single mode optical power output to beemitted through an emission area which is greater than laser diodesknown heretofore, which thus leads to an emitted beam which is of largercross-sectional dimensions than can otherwise be achieved. The singlespatial mode nature of the emission facilitates fibre coupling to thelaser diodes, and provides for a reduction in the size of the minimumspot to which the beam can be focused without loss of intensity. Afurther advantage of the laser diodes according to the invention is thatby virtue of the fact that lasing takes place in the wave guiding regionof lower refractive index, optical absorption at the emitting facet isreduced. These, thus, lead to both a high laser efficiency and improvedupper emitted power limits before the onset of optical damage at thefacet.

A further advantage of the laser diodes according to the invention isthat beam wander of the laser light output is restricted due to thediscrimination against lasing in the lower order modes of the lightpropagating layers which in the case of the laser diode of oneembodiment of the invention is determined by the placement of the gainmeans, in other words, the electrical current pumped quantum wells, andthe consequent high confinement factors of the lasing mode.

The invention will be more clearly understood from the followingdescription of some preferred embodiments thereof, which are given byway of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser diode according to theinvention,

FIG. 2(a) is a transverse cross-sectional end elevational view of thelaser diode of FIG. 1 on the line II—II of FIG. 1,

FIG. 2(b) is a graphical representation of the refractive index profileof respective layers of the laser diode of FIG. 1,

FIG. 3 illustrates waveforms of optical modes generated in the laserdiode of FIG. 1,

FIG. 4 illustrates a waveform of light output plotted against currentfor the laser diode of FIG. 1,

FIG. 5 illustrates waveforms which show a comparison between themeasured and calculated near field light emission pattern of the laserdiode of FIG. 1,

FIG. 6 illustrates waveforms of far field light emission patterns in thedirection parallel to the growth direction for the laser diode of FIG. 1for various laser operating currents,

FIG. 7 is a perspective view of a laser diode according to anotherembodiment of the invention,

FIG. 8 is a transverse cross-sectional end elevational view similar tothat of FIG. 2(a) of a laser diode according to another embodiment ofthe invention, and

FIG. 9 illustrates waveforms of intensity of light output of the laserdiode of FIGS. 7 and 8 and a conventional laser diode for comparisonpurposes.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings and initially to FIGS. 1 to 6 thereof, thereis illustrated a semiconductor laser diode according to the inventionindicated generally by the reference numeral 1 for outputting a highpower single mode light beam. The laser diode 1 extends longitudinallybetween a pair of mirror end facets 2 formed when the laser diode 1 iscleaved from a semiconductor wafer, at least one of which ends definesan aperture 4 through which the single mode light is outputted. Thelaser diode 1 comprises a substrate layer 5 of GaAs and a cladding layer6. A compound light propagating layer 7 is located between the substratelayer 5 and the cladding layer 6 and comprises a plurality of layerswhich are sequentially grown on the substrate layer 5, namely, a firstlayer 9, a second layer 10, a third layer 11 and a fourth layer 12, seeFIG. 2(a). The first layer 9 is of depth 2 μm while the fourth layer 12is of 1.3 μm depth, and both are of Al_(0.30)Ga_(0.70)As. The secondlayer 10 is of depth 3.15 μm and is of GaAs. The third layer 11 is ofdepth 1.2 μm and is of Al_(0.27)Ga_(0.73)As. The refractive index of thesecond layer 10 is relatively high, having a refractive index ofapproximately 3.52, see FIGS. 2(b) and 3. The refractive index of thethird layer 11 is deliberately chosen to be low at approximately 3.38,and the highest intensity anti-node of the light of a single spatialmode is propagated in the third layer 11. The wave guiding properties ofthe light propagating layer 7 are predetermined by virtue of thematerial and depth of the first to the fourth layers 9 to 12, and lightof a desired single mode is guided in the third layer 11.

A means for defining a wave guiding region 15 in the third layer 11within which the light of a single mode is guided, and for amplifyingthe intensity of the single mode light in the wave guiding region 15comprises a gain means, which in this embodiment of the invention isprovided by two quantum wells 16 which are formed in the third layer 11of InGaAs. The quantum wells 16 are located in the third layer 11 at aposition at which the highest order mode of the single mode lightoccurs, and at the anti-node of the single mode light, so that thehighest order mode is propagated and amplified as the dominant mode inthe wave guiding region 15 in preference to the other modes of the lightpropagating layer 7. As well as locating the quantum wells 16 at theposition where the anti-node of the highest order mode occurs, thequantum wells 16 are located in such a way as to ensure that the highestorder mode has the largest overlap with the quantum wells 16. Thisoverlap is known as the confinement factor ┌ for the propagating singlemode light.

A locating means for locating the wave guiding region 15 transverselyrelative to the direction of light propagation comprises an elongatedcentral locating ridge 17 which extends longitudinally along thecladding layer 6 in the direction of light propagation. A pair of spacedapart side channels 18 formed in the cladding layer 6 form the centralridge 17. An electrically conductive layer 19 is laid down on thecentral ridge 17 for facilitating pumping of a pumping electricalcurrent through the laser diode 1.

Referring now to FIG. 3, waveform A is a plot of the refractive indicesof the light propagating layer 7 and the cladding layer 6 which isplotted against distance from a centre point 20 of the first layer 9. Inother words, the waveform A of FIG. 3 is similar to the graphicalrepresentation of the refractive indices of the light propagating layer7 and the cladding layer 6 of FIG. 2(b). Waveforms B to H of FIG. 3illustrate the intensities which are plotted in arbitrary units of themodes of the light propagated in the first and second layers 9 and 10 ofthe compound light propagating layer 7. Waveform J illustrates theintensity of the highest order mode also plotted in arbitrary unitswhich is propagated in the wave guiding region 15 adjacent the quantumwells 16. The confinement factors relative to the various modes of lightare indicated in the right-hand column, and as can be seen, theconfinement factor ┌ resulting from the quantum wells 16 in the waveguiding region 15 is 3.36%, which thus ensures significant amplificationof the highest order mode at the expense of the other remaining modes.The value of the confinement factor ┌ for the single mode lightillustrated by the waveform J which has an anti-node with which thequantum wells 16 coincide is significantly higher than the value of theconfinement factor ┌ for any of the other modes in the first and secondlayers 9 and 10. In fact, as can be seen, the next confinement factor ┌below the confinement factor ┌ of the highest order single mode light is0.23%, which is more than a factor of ten less than the confinementfactor of 3.66% of the highest order single mode light.

The region 15 which is of lower refractive index than that of the secondlayer 10 in the Y direction, see FIG. 2(a), perpendicular to thedirection of light propagation, namely, in the direction of growth ofthe layers 9 to 12 of the light propagating layer 7 in this embodimentof the invention is determined by the actual refractive index of thematerial of the wave guiding region 15. The central ridge 17 increasesthe refractive index of the third layer 11 beneath the central ridge 17thereby centrally locating the wave guiding region 15 beneath thecentral ridge 17 in the X direction perpendicular to the Y direction,and also perpendicular to the direction of light propagation in the waveguiding region 15.

In this embodiment of the invention the central ridge 17 formed in thecladding layer 6 is 50 μm wide in the X direction, and the laser diodeis cleaved to a length in the direction of light propagation ofapproximately 1,000 μm. The laser diode 1 has a threshold current ofapproximately 170 mA corresponding to a threshold current density of340A/cm² or 170A/cm² for each quantum well 12. The low threshold currentof the laser diode 1 provides for operation in a mode with large overlapbetween it and the gain medium. FIG. 4 illustrates a waveform of outputoptical power in Watts plotted against current in Amps.

Referring now to FIG. 5, two waveforms of the near field emissionintensity pattern of the light of the laser diode 1 in arbitrary unitsplotted against distance in the Y direction, namely in the direction ofgrowth of the layers of the light propagating layer 7 are illustratedwith the zero point on the X axis taken as the position of the quantumwells 12 in the light propagating layer 7. The waveform A illustrated bya continuous line is the theoretical calculated value of the lightintensity of the near field emission pattern, while the waveform Billustrated by a broken line is the measured light intensity of the nearfield emission pattern. As can be seen the experimentally measured nearfield emission pattern agrees with the calculated pattern, demonstratingthat in this embodiment of the invention the laser mode propagating inthe laser wave guide is preferentially confined to the low index waveguiding region defined in the direction of growth of the laser diode.

FIG. 6 illustrates waveforms of light intensity in arbitrary units ofthe far field emission patterns plotted against angles in degrees forseveral operating currents from 200 mA to 1A. The zero angle position isarbitrary. The waveforms demonstrate that for this wide range ofoperating conditions the position and shape of the emitted mode isstable and does not exhibit beam wander. The laser diode 1 whenoperating at a current of 1A provides an optical power output of 300 mW,approximately. From a comparison of FIGS. 5 and 6, it can be seen thatthere is little change in the far field caused by increasing currentoutput, and thus, the laser diode according to the invention provides ahighly stable optical output.

Referring now to FIGS. 7 to 9, there is illustrated a laser diode 30also according to the invention. In this embodiment of the invention thelaser diode 30 comprises a substrate layer 32, a top cladding layer 33and a lower cladding layer 34. A light propagating layer 35 is locatedbetween the top cladding layer 33 and the lower cladding layer 34.However, in this embodiment of the invention the means for defining awave guiding region 36 in the light propagating layer 35 comprises aformation in the top cladding layer 33, namely, a longitudinallyextending central channel 37 extending in the direction of lightpropagation which is formed in a central locating ridge 38 in the topcladding layer 33. The central channel 37 defines the wave guidingregion 36 to be of lower refractive index than the adjacent regions inthe light propagating layer 35 in the X axis direction. An electricallyconductive layer 41 is provided on the surface of the bottom of thecentral channel 37 for facilitating pumping a current through the waveguiding region 36. The central channel 37 effectively acts as a gainmeans for amplifying light propagating in the wave guiding region 36 inpreference to other modes propagating in the light propagating layer 35.The central ridge 38 determines an active region 39 in the lightpropagating layer 36 in the X axis direction transversely of thedirection of light propagation in the light propagating layer 35, whilethe central channel 37 defines the low refractive wave guiding region 36in the X axis direction. The laser diode 30 extends between cleaved ends42 one of which defines an aperture 43 through which the single modelight is outputted.

Referring now to FIG. 9, waveform M illustrates the refractive indicesof the light propagating layer 35 in the X direction with the zero point40 of the X axis corresponding to the centre of the central channel 37in the X direction. The graph N is a plot of the light intensity of thehighest order mode which propagates in the wave guiding region 36 in theX direction against distance from the centre point 40 of the centralchannel 37. For the purpose of comparison, the light intensity plottedin arbitrary units against distance from the centre point of a waveguiding layer 35 of a laser diode which is similar to the laser diode 30but with the exception that the central channel 37 has been omitted areillustrated by the waveforms P and Q. In such a laser diode theeffective refractive index of the light propagating layer whichcorresponds to the light propagating layer 35 is constant, and higherthan the effective refractive index of the cladding layer, and thus thelowest order mode which is illustrated by the graph Q gives a relativelybroad lobed emission, while the highest order modes are attenuatedrelative to the lowest order mode, see waveform P. Accordingly, it canbe seen that by providing the central channel 37 for reducing theeffective refractive index of the light propagating layer 35 to definethe wave guiding region 36 causes the highest order mode to be largelyconfined beneath the central channel 37, thus giving a predominantlysingle lobed emission from the laser diode.

Table 1 sets out the materials, depths and refractive indices of therespective substrate layer 32, the lower cladding layer 34, the lightpropagating layer 35 and the top cladding layer 33 of the laser diode30. Table 2 sets out the depths T1 to T3 of the top cladding layer 33and the width W1 of the central ridge 38 and the width W2 of the centralchannel 37.

TABLE 1 Details of the layer structure of the laser diode 30 Refractiveindex Layer Material Depth (μm) (λ - 989 nm) Substrate GaAs — 3.5253Lower Cladding Layer A1_(0.3)Ga_(0.7)As 2.0 3.3603 Wave Guiding LayerGaAs 0.3 3.5253 Top Cladding Layer A1_(0.3)Ga_(0.7)As 3.3603

TABLE 2 Dimensions of top cladding layer of the laser diode 30 Dimensionμm t1 0.250 t2 0.750 t3 0.265 w1 61 w2 15

While laser diodes of layers of specific materials and specificdimensions have been described, it will be readily apparent to thoseskilled in the art that laser diodes according to the invention may beprovided with layers of other materials and other dimensions.

While in the embodiment of the invention described with reference toFIGS. 7 to 9 the means for defining the wave guiding region 36 isprovided by a formation 37 in the cladding layer 33, it is envisagedthat the means for defining the wave guiding region may be provided byother means. One such means for forming the wave guiding region would beto implant a longitudinally extending portion of the cladding layer sothat the implanted portion would extend longitudinally in the directionof light propagation, and in implanting the portion of the claddinglayer, the refractive index of the implanted portion would be reduced.Additionally, the implanted portion may be made electrically conductivefor pumping the wave guiding region. It is envisaged that the claddinglayer 33 could be provided as GaAsAlAs in the form of a super lattice.The elongated portion of the cladding layer extending in the directionof light propagation would then be implanted with zinc, which would thusform an electrically conductive portion of reduced refractive index.This would have a substantially similar effect in forming the elongatedchannel 37 in the cladding layer 33 as described in the embodiment ofthe invention with reference to FIGS. 7 to 9.

1. A laser diode for outputting light of a single spatial high ordermode, the laser diode comprising: a substrate layer, a cladding layer, alight propagating layer located between the substrate layer and thecladding layer, and a gain means comprising at least one quantum wellprovided in the light propagating layer for defining a wave guidingregion therein, a means being provided in one of the layers adjacent thelight propagating layer for reducing the refractive index of the definedwave guiding region to be lower than the refractive index of an adjacentregion in the light propagating layer in order that light of the highestorder single spatial mode is propagated in the defined wave guidingregion, so that when the defined wave guiding region is pumped theintensity of the light of the highest order single spatial mode isamplified above light of other lower order modes propagated in the lightpropagating layer.
 2. A laser diode as claimed in claim 1 in which thegain means comprises at least two quantum wells located in the lightpropagating layer.
 3. A laser diode as claimed in claim 1 in which thegain means comprises a plurality of quantum wells located in the lightpropagating layer.
 4. A laser diode as claimed in claim 1 in which thewave guiding region is defined in the light propagating layer duringgrowing of the layers of the laser diode.
 5. A laser diode as claimed inclaim 1 in which the means in one of the layers adjacent the lightpropagating layer for reducing the refractive index in a region in thelight propagating layer for forming the wave guiding region comprises aportion which is implanted in the one of the said layers such that theimplanted portion is of reduced refractive index to that of the said oneof the said layers.
 6. A laser diode as claimed in claim 1 in which themeans in one of the layers adjacent the light propagating layer forreducing the refractive index in a region in the light propagating layerfor forming the wave guiding region comprises a formation in the one ofthe said adjacent layers.
 7. A laser diode as claimed in claim 6 inwhich the formation is formed in the cladding layer.
 8. A laser diode asclaimed in claim 6 in which the formation in the layer adjacent thelight propagating layer comprises a central channel extendinglongitudinally through the said adjacent layer in the direction of lightpropagation in the light propagating layer for defining the wave guidingregion.
 9. A method for outputting light of a single spatial high ordermode from a laser diode, wherein the laser diode comprises a substratelayer, a cladding layer, and a light propagating layer located betweenthe substrate layer and the cladding layer, the method comprising thesteps of: providing a gain means comprising at least one quantum well inthe light propagating layer for defining a wave guiding region therein,providing a means in one of the layers adjacent the light propagatinglayer for reducing the refractive index of the defined wave guidingregion to be lower than the refractive index of an adjacent region inthe light propagating layer in order that light of the highest ordersingle spatial mode is propagated in the defined wave guiding region,and pumping a current through the defined wave guiding region so thatthe intensity of the light of the highest order single spatial mode isamplified above light of other modes propagated in the light propagatinglayer.
 10. A method as claimed in claim 9 in which the gain means isprovided by at least two quantum wells located in the light propagatinglayer.
 11. A method as claimed in claim 9 in which the gain means isprovided by a plurality of quantum wells located in the lightpropagating layer.
 12. A method as claimed in claim 9 in which the waveguiding region is defined in the light propagating layer during growingof the layers of the laser diode.
 13. A method as claimed in claim 9 inwhich the means in one of the layers adjacent the light propagatinglayer for reducing the refractive index in a region in the lightpropagating layer for forming the wave guiding region comprises aportion which is implanted in the one of the said layers such that theimplanted portion is of reduced refractive index to that of the said oneof the said layers.
 14. A method as claimed in claim 9 in which themeans provided in the said one of the layers adjacent the lightpropagating layer for reducing the refractive index in a region in thelight propagating layer for forming the wave guiding region is providedby a formation being provided in the said one of the said adjacentlayers.
 15. A method as claimed in claim 14 in which the formation isformed in the cladding layer, and an electrically conductive layer isformed on the formation in the cladding layer defining the wave guidingregion for facilitating pumping of the wave guiding region.
 16. A methodas claimed in claim 14 in which the formation in the layer adjacent thelight propagating layer comprises a channel extending longitudinallythrough the said adjacent layer in the direction of light propagation inthe light propagating layer for defining the wave guiding region.
 17. Alaser diode for outputting light of a single spatial high order mode,the laser diode comprising: a substrate layer, a cladding layer, a lightpropagating layer located between the substrate layer and the claddinglayer, and a gain means for defining a wave guiding region in the lightpropagating layer, the defined wave guiding region being of refractiveindex lower than an adjacent region in the light propagating layer andbeing defined in a region of the light propagating layer adjacent ananti-node of the highest order single spatial mode light propagated inthe light propagating layer, so that when the defined wave guidingregion is pumped the intensity of the light of the highest order singlespatial mode is amplified above light of other lower order modespropagated in the light propagating layer.
 18. A method for outputtinglight of a single spatial high order mode from a laser diode, whereinthe laser diode comprises a substrate layer, a cladding layer, and alight propagating layer located between the substrate layer and thecladding layer, the method comprising the steps of: providing a gainmeans in the light propagating layer for defining a wave guiding regiontherein, the defined wave guiding region being of refractive index lowerthan an adjacent region in the light propagating layer and being definedin a region of the light propagating layer adjacent an anti-node of thehighest order single spatial mode light propagated in the lightpropagating layer, and pumping a current through the defined waveguiding region so that the intensity of the light of the highest ordersingle spatial mode is amplified above light of other modes propagatedin the light propagating layer.
 19. A laser diode for outputting lightof a single spatial high order mode, the laser diode comprising: asubstrate layer, a cladding layer, a light propagating layer locatedbetween the substrate layer and the cladding layer, a gain meanscomprising at least one quantum well provided in the light propagatinglayer for defining a wave guiding region therein, the defined waveguiding region being of refractive index lower than an adjacent regionin the light propagating layer and being defined in the lightpropagating layer in a region in which light of the highest order singlespatial mode is propagated, so that when the defined wave guiding regionis pumped the intensity of the light of the highest order single spatialmode is amplified above light of other lower order modes propagated inthe light propagating layer, and a locating means for locating the waveguiding region in a direction transversely of the direction of lightpropagation.
 20. A laser diode as claimed in claim 19 in which thelocating means comprises a longitudinally extending central locatingridge formed on the cladding layer extending in the direction of lightpropagation for locating and stabilising the wave guiding region in thedirection transversely of the direction of light propagation, anelectrically conductive layer being provided on the central locatingridge for facilitating pumping of current through the wave guidingregion.
 21. A method for outputting light of a single spatial high ordermode from a laser diode, wherein the laser diode comprises a substratelayer, a cladding layer, and a light propagating layer located betweenthe substrate layer and the cladding layer, the method comprising thesteps of: providing a gain means comprising at least one quantum well inthe light propagating layer for defining a wave guiding region therein,the defined wave guiding region being of refractive index lower than anadjacent region in the light propagating layer and being defined in thelight propagating layer in a region in which light of the highest ordersingle spatial mode is propagated, forming a central locating ridge onthe cladding layer for locating the wave guiding region in a directiontransversely of the direction of light propagation, forming anelectrically conductive layer on the central locating ridge forfacilitating pumping of current through the wave guiding region, andpumping a current through the defined wave guiding region through theelectrically conductive layer so that the intensity of the light of thehighest order single spatial mode is amplified above light of othermodes propagated in the light propagating layer.